This page is just a reference of all the known Steam APIs, I do not know how they work. Please do not email me with questions.
UNDOCUMENTEDGetAssetClassInfo
https://api.steampowered.com/IGameInventory/GetAssetClassInfo/v1/
UNDOCUMENTEDGetAssetPrices
https://api.steampowered.com/IGameInventory/GetAssetPrices/v1/
UNDOCUMENTEDGetContexts
https://api.steampowered.com/IGameInventory/GetContexts/v1/
https://partner.steam-api.com/IGameInventory/GetHistoryCommandDetails/v1/
NameValueTypeRequiredDescription
uint32 Yes appid of game
uint64 Yes The steam ID of the account to operate on
string Yes The command to run on that asset
uint64 Yes The context to fetch history for
string Yes The arguments that were provided with the command in the first place
UNDOCUMENTEDGetItemDefArchive
https://api.steampowered.com/IGameInventory/GetItemDefArchive/v1/
https://partner.steam-api.com/IGameInventory/GetUserHistory/v1/
NameValueTypeRequiredDescription
uint32 Yes appid of game
uint64 Yes The Steam ID to fetch history for
uint64 Yes The context to fetch history for
uint32 Yes Start time of the history range to collect
uint32 Yes End time of the history range to collect
https://partner.steam-api.com/IGameInventory/HistoryExecuteCommands/v1/
NameValueTypeRequiredDescription
uint32 Yes appid of game
uint64 Yes The asset ID to operate on
uint64 Yes The context to fetch history for
uint32 Yes A unique 32 bit ID for the support person executing the command
https://partner.steam-api.com/IGameInventory/SupportGetAssetHistory/v1/
NameValueTypeRequiredDescription
uint32 Yes appid of game
uint64 Yes The asset ID to operate on
uint64 Yes The context to fetch history for
UNDOCUMENTEDPOSTUpdateItemDefs
https://api.steampowered.com/IGameInventory/UpdateItemDefs/v1/
NameValueTypeRequiredDescription
click to setstring Yes Steamworks Web API publisher authentication key.
uint32 Yes appid of game
JSON array Yes One or more Item Definitions, presented as a JSON array, to be updated or created.